An antiferroelectric liquid crystal is stable in an antiferroelectric state when left in a condition that no voltage (zero) is applied to the liquid crystal. Hereinafter, this stable state will be referred to as a neutral state. An antiferroelectric liquid crystal panel may be configured in such a manner as to effect either a dark display or a bright display in the aforesaid neutral state. An antiferroelectric liquid crystal panel of the present invention be applied to both a dark display and a bright display. An antiferroelectric liquid crystal panel which is adapted to effect a dark display in the neutral state, will be described in the following description. However, in the case of an antiferroelectric liquid crystal panel which is adapted to effect a bright display in the neutral state, read "bright" for "dark", and vice versa in the following description.
It is explained that generally, an antiferroelectric liquid crystal has two states, namely, an antiferroelectric state (namely, a dark state) and a ferroelectric state (namely, a bright state), that when a voltage is applied to a liquid crystal panel, the entirety of which has been in the antiferroelectric state, a phase transition therefrom to the ferroelectric state first occurs in micro portions, that the proportion of portions, whose phases are changed into the ferroelectric state, increases with time and that finally the phase of the entire panel is changed into the ferroelectric state and thus the entire panel changes the state thereof into a saturated ferroelectric state.
When zero voltage is applied to a liquid crystal panel, the entirety of which has been in the ferroelectric state, in a similar way, a phase transition therefrom to the antiferroelectric state first occurs in micro portions, that the proportion of portions, whose phases are changed into the antiferroelectric state, increases with time and that, finally, the phase of the entire panel is changed into the antiferroelectric state and thus the entire panel changes the state thereof into the neutral state.
FIG. 1(a) is an example of a graph illustrating the optical transmittance of an antiferroelectric liquid crystal versus a voltage applied thereto. In this graph, the axis of abscissa represents the applied voltage; and the axis of ordinates represents the optical transmittance.
When applying a positive voltage to the crystal, which has been in the neutral state at a point O, and increasing the positive voltage, the transmittance abruptly increases at a voltage Ft. Then, the transmittance reaches nearly the maximum value at a voltage Fs, and the crystal changes the state thereof into the saturated ferroelectric state. Thence, the optical transmittance does not change much even when a higher voltage is applied thereto. Next, when the applied voltage is gradually decreased, the optical transmittance abruptly drops at a voltage At. Further, the transmittance nearly reaches zero at the voltage As, and the state of the crystal returns to an antiferroelectric state. Similarly, if a negative voltage is applied to the crystal from the voltage 0, the transmittance abruptly rises at a voltage (-Ft). Then, the transmittance nearly reaches the maximum value at a voltage (-Fs), and the crystal changes the state thereof into the saturated ferroelectric state. Thence, when the applied negative voltage is gradually reduced to 0 V, the transmittance abruptly drops at a voltage (-At). Further, the transmittance becomes almost zero at a voltage (-As), and, the crystal returns to the antiferroelectric state. As above described, there are two ferroelectric states of the liquid crystal, which are the ferroelectric state caused by the application of the positive voltage and the ferroelectric state caused by the application of the negative voltage. Hereunder, the ferroelectric state caused in the former case will be referred to as the (+) ferroelectric state, while the ferroelectric state caused in the latter case will be referred to as the (-) ferroelectric state. Further, .vertline.Ft.vertline. designates a ferroelectric threshold voltage; .vertline.FS.vertline. a ferroelectric saturation voltage; .vertline.At.vertline. designates an antiferroelectric threshold voltage; and .vertline.As.vertline. an antiferroelectric saturation voltage.
Incidentally, the values of each of the aforementioned ferroelectric threshold voltage .vertline.Ft.vertline., the aforementioned ferroelectric saturation voltage .vertline.FS.vertline., the aforementioned antiferroelectric threshold voltage .vertline.At.vertline. and the aforementioned antiferroelectric saturation voltage .vertline.As.vertline. in the (+) ferroelectric side is sometimes slightly different from the value thereof in the (-) ferroelectric side. However, for simplicity of description, the following description will be made by assuming that each of these voltage has the same value. To make a correction on the driving voltages respectively corresponding to the (+) ferroelectric side and the (-) ferroelectric side as needed is within the scope of the present invention.
Generally, it is the case that the curves (namely, hysteresis curves) of FIG. 1(a) representing the optical transmittance characteristics of a liquid crystal versus the voltage applied thereto are obtained by applying thereto a triangular-wave-like voltage in which the absolute value of the ratio of a change in this voltage relative to time, namely, the value of .vertline.dV/dt.vertline. is constant. However, in this case, if the value of .vertline.dV/dt.vertline. is changed, the shapes of the hysteresis curves also change. Moreover, the values of the aforementioned values As, Ft, Fs and At also vary. It is, accordingly, necessary to define the aforesaid value of .vertline.dV/dt.vertline. to specify these values. However, the inventors obtained data concerning the graph of FIG. 1(a) by the following method (hereunder referred to as a time fixation method 1) so as to obtain values closely corresponding to actual driving conditions.
It is assumed that the length of a time period, in which a selection voltage (to be described later) is applied to a display device at a working temperature, is Wt.
(1) A pulse voltage, whose duration is Wt and voltage level is Vx, is applied to the liquid crystal that is in a stable antiferroelectric state (namely, in the neutral state). Further, the relationship between the optical transmittance and the pulse voltage Vx at the time of completion of the application of this pulse voltage is plotted. When this operation is repeated by changing the value of the voltage Vx, the curve drawn from the point O to Fs through Ft of FIG. 1(a), as well as the curve drawn from the point O to (-Fs) through (-Ft), is obtained. PA1 (2) Next, the liquid crystal is first put into the saturated ferroelectric state by applying thereto a voltage which is not lower than the aforementioned voltage .vertline.FS.vertline.. Then, at a moment 0, the applied voltage is reduced to .vertline.Vz.vertline.. Thence, after the elapse of the assumed relaxation period (to be described later), the relationship between the value of the optical transmittance and the applied voltage Vz is plotted. When this operation is repeated by changing the value of the voltage .vertline.Vz.vertline., the curve drawn from Fs to the point O through At and As of FIG. 1(a) and the curve drawn from (-Fs) to the point O through (-At) and (-As) are obtained.
In the case of using some liquid crystal panels, the curve (namely, the curve drawn from Fs or (-Fs) to the point O in FIG. 1(a)) obtained in the aforementioned case (2) sometimes intersects the ordinate axis. The main cause of this is the responsivity of the liquid crystal. Namely, in the case that the liquid crystal is maintained in the ferroelectric state by applying thereto a voltage, which is not lower than the aforementioned voltage .vertline.FS.vertline., and that at the moment 0, the applied voltage Vz is changed into 0, the liquid crystal finally becomes stable in the antiferroelectric state after the elapse of a certain time period (hereunder referred to as a relaxation time tn). However, if this relaxation time tn is longer than the aforementioned relaxation period, the curve obtained in the aforementioned case (2) intersects the ordinate axis.
When actually driven, it is difficult to bring such a liquid crystal panel into a complete antiferroelectric state. It is, thus, considered that in the case of such a liquid crystal panel, a dark display cannot be effected and that consequently, the contrast is extremely degraded.
Generally, a liquid crystal panel is driven by performing the following process. Namely, first N row electrodes and M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. Further, a scanning signal is applied to each of the row electrodes through a row-electrode drive circuit, a display signal depending on display data of each pixel (incidentally, a part of the display signal is sometimes not dependent on the display data) is applied to each of the column electrodes through a column-electrode drive circuit. Moreover, a voltage (hereunder referred to simply as a synthetic voltage), which corresponds to the difference between the scanning signal and the display signal, is applied to a liquid crystal layer. The time period required to scan all of the row electrodes (namely, 1 vertical scanning interval) is usually referred to as 1 frame (or 1 field). In the case of driving the liquid crystal panel, the polarity of a driving voltage is reversed each frame (or every frames) in order to prevent the liquid crystal from being adversely affected (for example, the degradation due to the nonuniform distribution of ions).
FIG. 2 illustrates the waveforms of signals flowing through the row electrodes, the column electrodes and the pixel synthesis electrodes of a liquid crystal panel in which the N row electrodes and the M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. The display states of pixels are assumed as follows. Namely, in the case of a first column (Y1), pixels in all rows are displayed in white. Further, in the case of a second column (Y2), a pixel in a first row is displayed in black, and pixels in the other rows are displayed in white. Moreover, in the case of pixels in a third column (Y3), these pixels are displayed alternately in black and in white. Furthermore, in the case of an Mth column, YM pixels are displayed in black in all rows.
Waveforms of scanning signals are respectively applied to the N row electrodes in sequence from the top row to the bottom row so that each of the waveforms is shifted by a time (1/N). Waveforms of display signals applied to the M column electrodes are synchronized with the scanning signal and the waveforms according to whether the pixels are displayed in white or in black are applied.
Paying attention to a synthetic voltage applied to each pixel, with respect to P11 displayed in white and P12 displayed in black in the first row, the voltage applied to P11 in the selection period tw, which is displayed in white, is a large waveform, whereas the voltage applied to P12 in the selection period tw, which is displayed in black, is a small waveform. The voltage applied to a pixel P21, which is displayed in white in a second row has a waveform which is almost the same as that obtained by shifting the waveform of the synthetic voltage applied to the pixel P11 by (1/N). Here, note that the first and the second frame in the first row and the second row are shifted with respect to each other by (1/N).
Turning attention to a scanning signal applied to a single row electrode, 1 vertical scanning interval thereof is composed of N horizontal scanning intervals (in some cases, an additional period .alpha. is added thereto). In a horizontal scanning interval, a part of a horizontal scanning interval in which a special scanning voltage (set at the selection voltage) to be used for determining the display state of a pixel on the aforesaid row is applied, is referred to as a selection period tw. The other part of horizontal scanning interval is referred to as non-selection periods. Moreover, generally, the selection period tw is a time period obtained by dividing 1 frame interval by (N+.alpha.).
Usually, in the case of the antiferroelectric liquid crystal panel, it is determined on the basis of the aforementioned display signal at the time of applying the selection voltage whether the state of the liquid crystal, which has been in the antiferroelectric state, is maintained or is changed into the ferroelectric state. Thus, there is the necessity of a time period (hereunder referred to as a relaxation period ts) required for setting the liquid crystals in the antiferroelectric state before the application of the selection voltage. During a time period which is other than the selection period tw and the relaxation period ts, the determined state of the liquid crystal should be held. Hereunder, this time period will be referred to as a holding period tk.
FIG. 3 illustrates the waveforms of a scanning signal waveform Pa applied to a given pixel of interest, display signal waveforms (Pb, Pb'), synthetic signal waveforms (Pc, Pc') and optical transmittances (L100, L0) according to a driving method described in FIGS. 1 and 2 of the Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992. F1 and F2 designate the first frame F1 and the second frame F2, respectively. Although not shown in this figure, a scanning signal, which has a phase shifted by 1 horizontal scanning interval and has a waveform similar to the scanning signal Pa or has a waveform obtained by inverting the polarity of the scanning signal Pa, is applied to a row electrode adjacent to a row electrode of interest.
FIG. 3 illustrates the case where the polarity of the aforementioned driving voltage is inverted every frame. As is apparent from this figure, the first frame Fl is different from the second frame F2 only in that the polarity of the driving voltage is inverted. As is obvious from the aforementioned FIG. 1(a), an operation of the liquid crystal display device is symmetric with respect to the polarity of the driving voltage. Therefore, the following description will be given regarding only the first frame, except in case of necessity. Further, in the following description and drawings of the waveforms of driving signals, the electric potential indicated as "0" does not mean absolute electric potential but means merely a reference electric potential. Therefore, in the case that the reference electric potential varies for some reason, scanning signals and display signals vary relatively. Moreover, in the case that the word "voltage" is used in connection with the scanning signals and the display signals in the following description, the word "voltage" designates the difference between the electric potential indicated by such a signal and the reference electric potential.
As shown in FIG. 3, 1 frame is divided into three time periods, namely, the selection period tw, the holding period tk and the relaxation period ts. The selection period tw is further divided into time periods tw1 and tw2, which have equal lengths. The voltage level of a scanning signal Pa in the first frame F1 is set as follows. Needless to say, in the second frame F2, the polarity of the voltage is inverted. Here, note that .+-.V1 designates the selection voltage and that the length of the time period tw2 corresponds to the aforementioned Wt.
______________________________________ Time Period tw1 tw2 tk ts ______________________________________ Scanning Signal Voltage 0 +V1 +V3 0 ______________________________________
Further, the display signal is set as follows. Here, note that the voltages indicated by the symbol "*" depend on the display data of other pixels in a same column.
______________________________________ Time Period tw1 tw2 tk ts ______________________________________ On-State Display Signal Voltage +V2 -V2 * * Off-State Display Signal Voltage -V2 +V2 * * ______________________________________
In the time period in which the aforementioned selection voltage is applied, each liquid crystal pixel on the selected row is selectively driven on the basis of the display signal. Hereinafter, a time period, in which the scanning signal is the selection voltage, is referred to as a selective driving period (in the case of this conventional example, tw2).
A part, which actually controls a display based on the display data, of the display signal, is a part, which corresponds to the selective driving period, thereof. This part of the display signal is simultaneously applied to liquid crystal pixels arranged on rows which are other than the selected row (and correspond to either the holding period tk or the relaxation period ts in the case of this conventional example) and thus adversely affects the states of the unselected liquid crystal pixels.
For example, in the case that in the hysteresis curves of FIG. 1(a), the curve drawn from As to Ft or from At to Fs is not flat, when the voltage applied to the liquid crystal in the holding period tk is shifted depending on the display signals of the other rows, a change in the brightness in this holding period is caused.
Thus, to compensate for the ill effects of this phenomenon, the period tw1 is established in addition to the selective driving period tw2 and moreover, the polarity of the display signal is inverted in the periods tw1 and tw2 so that the average value thereof in a horizontal scanning interval is 0.
Namely, the display signal in the period tw1 plays a role in compensating for the ill effects of the display signal on the pixels, which are arranged on the unselected rows, in the selective driving period. Therefore, hereinafter, a time period, in which the display signal is used for such compensation, is referred to as a compensation signal period.
In FIG. 3, Pb, Pc and L100 respectively denote the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an on-state (namely, in the bright state). In this case, if the (synthetic) voltage to be applied to the liquid crystal in the selective driving period tw2 meets the following condition: .vertline.V1+V2.vertline.&gt;.vertline.Fs.vertline. (see FIG. 1(a)), the transition of the state of the liquid crystal into the ferroelectric state is started. As a result, the optical transmittance of the liquid crystal increases. In the holding period tk, if the following condition is satisfied: .vertline.V3-V2.vertline.&gt;.vertline.At.vertline., the bright state can be held. In the relaxation period ts, if the following condition is satisfied: .vertline.V2.vertline.&lt;.vertline.AS.vertline., the transmittance decreases with the elapse of time. Thus, the relaxation of the liquid crystal, by which the state thereof from the ferroelectric state is changed to the stable antiferroelectric state, is attained.
Further, in FIG. 3, reference characters Pb', Pc' and L0 respectively denote the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an off-state (namely, in the dark state). In this case, if the synthetic voltage to be applied to the liquid crystal in the selective driving period tw2 meets the following condition: .vertline.V1-V2.vertline.&lt;.vertline.Ft.vertline., the voltage applied in the holding period tk meets the following condition: .vertline.V3+V2.vertline.&lt;.vertline.Ft.vertline., and the voltage applied in the relaxation period ts meets the following condition: .vertline.V2.vertline.&lt;.vertline.Ft.vertline., the dark state is displayed.
FIG. 4 is a waveform diagram illustrating the waveform of a driving signal used in the driving method that is described in the Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994. In the case of this driving method, 1 frame is divided into the selection period tw and the holding period tk. The aforesaid selection period tw is further divided into three time periods, namely, two time periods tw1 and tw2, which have equal lengths, and a time period tw0 which precedes the two periods tw1 and tw2. In the case of this driving method, the aforementioned relaxation period ts is the aforesaid time period tw0. The length of the time period tw0 is not always equal to that of each of the time periods tw1 and tw2. Further, the voltage level of the scanning signal and the display signals in the first frame F1 are set as follows.
______________________________________ Time Period tw0 tw1 tw2 tk ______________________________________ Scanning Signal Voltage 0 0 +V1 +V3 On-State Display Signal Voltage 0 +V2 -V2 * Off-State Display Signal Voltage 0 -V2 +V2 * ______________________________________
In the case of the driving method described in this Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994, the zero-volt voltage applying time period (tw0) provided in the leading part of the selection period tw is used as the relaxation period ts. Further, the time period tw1 is the compensation signal period. Moreover, the time period tw2 is the selective driving period.
In the case of the two conventional examples, the selective driving periods, during each of which the selection voltage .vertline.V1 .vertline. is applied so as to determine the bright or dark state of the liquid crystal, are tw2. However, if the length of the aforesaid time period tw2 is insufficient, the transition of the state of the liquid crystal to the ferroelectric state cannot be sufficiently achieved. This impedes the display. Namely, in the case that the transition of the state of the liquid crystal, which has been in the stable antiferroelectric state and presented the dark state, to the almost saturated bright state is performed by applying a constant voltage thereto, a time period (hereunder referred to a ferroelectric saturation time tr) having a certain length is required. Therefore, if the period tw2 becomes shorter than the aforesaid ferroelectric saturation time tr, a change in optical transmittance results in a failure in presenting the bright state sufficiently as indicated by dashed lines in a graph L100 of FIG. 3, so that the contrast is degraded.
In the case of the driving method described in FIGS. 1 and 2 of the Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992, the length of the aforesaid selective driving period tw2 is a half of the selection period tw. In the case of the driving method described in the Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994, the length of the aforesaid selective driving period tw2 is less than a half of the selection period tw. Further, generally, the length of the aforesaid selection period tw is given by: EQU tw=F/(N+.alpha.)
where F denotes the length of 1 frame. Thus, increasing the length of the frame is sufficient to lengthen the period tw2. However, when F is longer than 20 ms (corresponding to 50 Hz), a flicker phenomenon occurs with the result that the display quality is deteriorated. Thus, there is a limit to the length of 1 frame. Under such a limitation, the length of the period tw (thus, the length of the selective driving period tw2) depends on (N+.alpha.). Therefore, the number N should be small so as to obtain the sufficient length of the period tw2.
The aforementioned ferroelectric saturation time tr changes according to the applied voltage and becomes short when the applied voltage is increased. Hence, when the applied voltage is large, the transition of the state of the liquid crystal to the ferroelectric state can be achieved even if the selective driving period tw2 is short. Usually, the aforementioned column electrode drive circuit and the aforementioned row electrode drive circuit have the maximum ratings, respectively. Consequently, voltages, which exceed the aforesaid ratings, cannot be supplied to the liquid crystals. Furthermore, there are driving restrictions on the selection voltage (.vertline.V1.vertline.) and the display signal voltage (.vertline.V2.vertline.). Thus, there is an upper limit to the voltage which can be applied to the liquid crystals.
Those restrictions result in occurrence of the upper limit to the number of row electrodes on the assumption that the length of the frame is constant. Consequently, it becomes difficult to provide a high resolution display device.
Moreover, the requirement of the large applied voltage results in increase in load imposed on the drive circuit and rise in the power consumption of the display device.